Only Human

To the Editors:

I especially enjoyed Pat Shipman’s column “Why Is Human Childbirth So Painful?” (Marginalia, November–December). As a former professor of gross anatomy, I have always subscribed to the “obstetrical dilemma hypothesis.” But after reading this column, I tend toward the alternative that childbirth has become more difficult as improved diet has increased newborn body size. Humans have enjoyed a calorie-rich diet from well before the dawn of what is called civilization. So one might have to go back to the early hominid Australopithecus to compare precivilization human brain, pelvis, and body sizes.

From 1976 to 1994 my primary National Institutes of Health grant was to investigate relationships between protein-calorie nutritional insufficiency and brain growth and development. My previous work has made me wary of broad conclusions based on measurements of humans’ growth, because the variation that one can observe among individuals is so huge. In the matter of body and brain size for age, we have no idea what is biologically “normal” for Homo sapiens. Pediatricians can show mothers growth charts, which are founded on a limited sample, but they can’t say what is normal. Somewhere at the intersections of physiology, anatomy, genetics, and physical anthropology, there are probably answers. I would love to see the brain sizes of Australopithecus, Homo erectus, Homo neanderthalensis, and other hominids plotted for comparison with Holly Dunsworth’s plots of brain size in modern humans and apes.

Regardless of the mother’s nutritional circumstances, I am aware that it is exceedingly difficult to undernourish a fetus in utero, but I do think an anthropological approach to the obstetrical–energetics debate would yield valuable information. Diet may be a key factor.

Richard Wiggins
Apex, NC

To the Editors:

I enjoyed reading Pat Shipman’s column concerning “Why Is Human Birth So Painful?” but was troubled by the anthropocentric approach and some factual errors. Dr. Shipman is wrong when she states, “Human newborns are unique among mammals [that give birth to a single young] in that our babies cannot immediately get up, feed, and walk around.” The order Chiroptera (bats) contains over 1,000 species, and about 90 percent give birth to only a single offspring at a time, as detailed in a paper that Thomas Kunz and I published in the Symposia of the Zoological Society of London in 1987. Although developmental state at birth varies among species, many bats are born blind, hairless, and helpless.

Dr. Shipman also is incorrect in stating that humans “bear large babies.” The offspring of bats are the largest of all mammals, relative to size of the mother. The mass of the average newborn bat is 22 percent of the mass of its mother postpartum, with some neonates weighing an incredible 43 percent of maternal mass. Human babies, in contrast, represent only a paltry 7 percent or less of their mothers’ mass.

Further, the hypothesis that humans are born in an underdeveloped state because the mother can no longer obtain the energy and nutrients for continued intra-uterine growth totally ignores the fact that lactation is energetically more demanding than pregnancy for virtually every eutherian mammal, from aardvarks to zebras. The demands of lactation are not unique to humans. Also, if a female bat can obtain energy and nutrients to create a fetus amounting to 43 percent of her mass, humans may be special and unique only in that, in many ways, their abilities fall short of what other mammals can do.